A labor union representing thousands of air traffic controllers sued the federal government on Friday because its members are working without pay during the shutdown.

The National Air Traffic Controllers Association filed its lawsuit on the 21st day of the partial government shutdown in the United States District Court for the District of Columbia, according to a statement.

The lawsuit also came on the first day that some 800,000 federal employees missed their paychecks since the start of the shutdown in December.

The union represents nearly 20,000 air traffic controllers, engineers and other aviation safety-related workers, according to its website. Many of its members were working without pay, the statement said, and approximately 3,000 members had been furloughed, meaning they were not working or receiving pay.

“NATCA alleges that the government unlawfully deprived NATCA members of their earned wages without due process,” the statement said.

The union is demanding that the government pays its members for work they’ve performed, as well as “liquidated damages.”

The Department of Transportation, which includes the Federal Aviation Administration and employs many of the NATCA's members, is one of the federal agencies that had not been funded. In a statement on Thursday, the FAA reiterated that safety is the agency’s top priority.

“Air traffic controllers and the technicians who maintain the nation’s airspace system continue to serve their critical mission to ensure the public’s safety,” the FAA’s statement said.